Iron and Thyroid: Why Low Iron Worsens Hypothyroid Symptoms
Low iron makes hypothyroidism worse because the enzyme that builds thyroid hormone, thyroid peroxidase, depends on iron to work. When stored iron runs low, TSH often drifts upward, conversion of T4 into active T3 slows, and symptoms such as fatigue, hair thinning and cold hands get heavier, even when the thyroid dose itself looks unchanged.
That is why treating iron and treating thyroid as two unrelated problems tends to leave people stuck. Both show up in the same lab report, and both produce a nearly identical symptom list.
How iron and thyroid are wired together
Three mechanisms explain the link, and all three are well described in the clinical literature.
- Thyroid peroxidase is a heme enzyme. It attaches iodine to tyrosine residues to build T4 and T3. Heme means it needs iron at its core. When iron is scarce, the enzyme cannot keep pace, and the thyroid has to work harder for the same output.
- Conversion of T4 to T3 drops. The deiodinase enzymes that convert stored T4 into active T3 are also iron dependent. Low iron can leave free T4 normal while free T3 sits at the bottom of the range, which is the pattern that produces symptoms despite "normal" thyroid numbers.
- TSH can rise in iron deficiency alone. Studies in iron deficient women without thyroid disease show higher TSH values that fall again once iron is repleted. This is one reason a mildly elevated TSH should not automatically lead to a lifelong prescription.
Why this pattern is so common in women
Iron deficiency is the most common nutrient deficiency worldwide, and the groups most affected overlap heavily with the groups most affected by thyroid problems.
- Menstruation. Heavy periods are the leading cause of iron deficiency in premenopausal women, and thyroid disease is also far more common in women.
- Pregnancy and postpartum. Iron demand rises sharply, and the postpartum period combines blood loss, poor sleep and often untreated thyroid changes.
- Hashimoto's thyroiditis. This autoimmune form of hypothyroidism is frequently accompanied by other autoimmune conditions, including coeliac disease and atrophic gastritis, both of which reduce iron absorption.
- Low stomach acid and acid reducers. Iron absorption depends on stomach acid, so long-term proton pump inhibitor use reduces it.
Why the symptoms are so easy to misread
Fatigue, brain fog, hair shedding, cold intolerance, brittle nails, poor exercise tolerance and low mood appear on both lists. Without a blood test there is no reliable way to tell which one is driving what, and in practice both are often active at once. Improving only one leaves the other half of the symptoms in place.
Which tests actually tell you something
| Test | What it shows | Practical note |
|---|---|---|
| Ferritin | Stored iron, the earliest and most sensitive marker | Ask for the number, not just "normal". Below 30 ng/mL is low even when the lab flags nothing. |
| Haemoglobin and full blood count | Whether iron deficiency has progressed to anaemia | Normal haemoglobin does not rule out low iron stores. |
| TSH | Pituitary signal to the thyroid | Rises in iron deficiency without thyroid disease. |
| Free T4 and free T3 | Hormone available for the body | A low free T3 with normal free T4 points toward a conversion problem. |
| Thyroid peroxidase antibodies | Autoimmune thyroid disease | Positive antibodies change how closely iron and thyroid need to be watched. |
Key takeaways
- Thyroid peroxidase and the T4 to T3 conversion enzymes both require iron, so low iron directly impairs thyroid hormone production.
- A mildly raised TSH in an iron deficient woman may reflect the iron deficiency, not thyroid failure.
- Ferritin is the marker to ask for. Below 30 ng/mL is low, and many clinicians aim higher for people with heavy periods or hair shedding.
- Iron and levothyroxine compete for absorption. Separate them by at least four hours.
- Coffee, tea, calcium and high-fibre foods block iron absorption. Vitamin C and animal protein improve it.
How iron, selenium and iodine divide the work
| Nutrient | Role in thyroid function | Where it comes from |
|---|---|---|
| Iron | Core of thyroid peroxidase; supports T4 to T3 conversion | Red meat, liver, shellfish, lentils, fortified cereals |
| Selenium | Required by deiodinase enzymes; part of the antioxidant defence of the thyroid gland | Brazil nuts, seafood, eggs, organ meats |
| Iodine | Raw material for T4 and T3; both deficiency and excess can disturb the gland | Iodised salt, dairy, seafood, seaweed |
This is a rare case where more is genuinely not better. Excess iodine can trigger thyroid dysfunction in susceptible people, and selenium doses above the upper limit carry their own risks. Iron, selenium and iodine all sit in a narrow working range.
Supplying iron sensibly
Form matters for tolerability as much as absorption. Ferrous sulfate is cheap and effective but causes constipation and nausea in a notable share of people. Iron bisglycinate is chelated and generally gentler on the stomach, which improves the odds that someone actually keeps taking it. If you are also on levothyroxine, take the iron at the opposite end of the day. Vitamin C taken at the same time increases absorption, while coffee, tea, milk and calcium supplements all reduce it. Repeat ferritin after about three months, since rebuilding stores takes longer than correcting haemoglobin.
Frequently asked questions
Can low iron cause hypothyroidism?
Iron deficiency does not destroy the thyroid gland, so it does not cause autoimmune hypothyroidism. It does impair thyroid hormone production and conversion, and can raise TSH on its own. Correcting the iron often improves those numbers.
What ferritin level should I aim for with thyroid symptoms?
Many clinicians aim for at least 50 ng/mL in people with fatigue or hair shedding, and at least 30 ng/mL as a floor. Ranges vary by laboratory and by clinical context, so discuss the target with the clinician managing your thyroid.
How far apart should I take iron and thyroid medication?
At least four hours. Iron binds levothyroxine in the gut and reduces how much you absorb. Taking the thyroid tablet on an empty stomach in the morning and the iron in the evening is the simplest way to separate them.
Do I need iron if my haemoglobin is normal?
Possibly yes. Haemoglobin falls late, after stores are already depleted. Fatigue, hair loss and exercise intolerance can appear while haemoglobin still reads normal, which is why ferritin is the more useful early test.
Related supplements
If you want to explore this area, the Vitamin B and Iron collections cover the relevant supplements and formats.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Talk to a healthcare professional before starting any new supplement, especially if you are pregnant, nursing, taking medication, or managing a health condition.
